The purpose of this thread is to make a reference list of differences between PrBoom+ complevels 9 and 2, in other words what Boom behaviours are different from classic Doom2 1.9 behaviours. I think there is no such a list created so far.

- dead bodies can pass through things, allowing monster wiggle death slides (building up speed by running against a monster, getting slashed to death and sliding the corpse to exit, potentially through the monster and under an otherwise impassible barrier)
- someone will need to verify this, but larger monsters seem to have smaller impassible hitboxes. it becomes easier to, say, walk around masterminds

Oh, I would have forgotten... below N_A's post there was Phml's, I'm not sure about #1 and #4, don't get #2, but I'll definitely add #3 to the first post (weapon switching)

- monsters not going back to sleep once they infought something outside of player view
- (all?) overflows fixed
- a new bug forcing a switch back to your previous weapon the first time you select a weapon out of ammo
- cyberdemons and spider masterminds not roaring everytime they get a new target

Edit: Hm... I tested it thoroughly on MAP30 from Doom II now. I cannot say what the exact Boom rule is. E.g. I have SG, SSG, plasma, no cells. If I keep shooting the SG, switch to plasma, I get an automatic switch to SSG (next best weapon with available ammo). The only difference is when you keep firing and switch to a no-ammo weapon too early if I'm not mistaken. Thus "different weapon switching beaviour" as I'm afraid to enter more details and put them wrongly.

dew said:- dead bodies can pass through things, allowing monster wiggle death slides (building up speed by running against a monster, getting slashed to death and sliding the corpse to exit, potentially through the monster and under an otherwise impassible barrier)

To clarify, the player corpse attains 0 height upon death in vanilla too, which is how Vincent Catalaa recorded pl17-009 back in 2001, using Final Doom v1.9 - see here on Doom Wiki. I'm not sure if other Things block a sliding player corpse and/or if that is actually what you meant.

DEH maxhealth in vanilla applies only to potions (the reverse of what is stated above), limiting the value to which potions can increase health (default = 200). Potions will also decrease your health to this value if you are above it (so you can become a zombie if it is set to zero).

In Boom, it is just screwed up, as the Boom team appeared to misunderstand what it did. Their default value is 100, and in Boom maxhealth defines the max value that stimpacks, medikits and berserks can increase health (this can't be changed from 100 in vanilla). As an unintended artefact (presumably), the limit for potions is twice the maxhealth value.

Note that this means that any choice of maxhealth value will have totally different effects in Boom vs vanilla. The only way to achieve the default behaviour in both vanilla and Boom with the same deh patch is not to include this value at all.

On the sliding through things point: the key difference is that in vanilla, you can only slide through gaps of at least a certain height (due to height reduction on death). The vanilla corpse cannot slide through things. The live player can pass through dead things, of course, provided they are not solid. With Boom behaviour, the dead player can pass through live things, and this has nothing to do with the height reduction.

Obviously, you'd need to go through the code to come up with a complete list - for instance everything covered under the "... exactly like Doom" comp options. There's a surprising amount of differences of many types covered there. (This, this and this may also be useful.) It might also be worth drawing a distinction between deliberate Boom changes that are optional (i.e. those specific to complevel 9) and non-optional changes in behaviour (those present in complevel 7 too). complevel 7 emulates Boom's attempt to emulate vanilla behaviour, and is what Boom 2.02 and Prboom 2.02 will use if asked to play back a vanilla demo.

In that vein, I'll mention that Boom follows Ultimate Doom in terms of lost soul behaviour (which will desync a lot of Doom2.exe demos) and does not feature Final Doom's teleporter bug (which will desync a lot of Final Doom demos).

I was going to link you to the Doom2.net page on this bug, but the website is down, so I guess I'll have to explain it myself. When doing blockmap calculations, an object's "bounding box" is effectively cut off by the borders between blocks in a blockmap. This is why you'll often see hitscan attacks go straight through larger monsters when it should have hit, and it makes it almost impossible to melee Arachnotrons and Mancubi.

I'm sure dew is well aware of the effect you are referring to. His suggestion was that Boom may have made some change (compared to vanilla) in the bounding boxes, or the way they interact, which is a totally different question.

I'm not aware of anything like that. You could test (to some degree) by recording vanilla demos that feature the alleged different behaviour and seeing if they play back in (Pr)Boom 2.02.

If there is anything, it could be due to changes in the friction code.

creaphis: i'm sorry, i meant wobble of course. i blame the late hour and my tired state when i wrote that. :)

wagi: what grazza said, i thought there was a difference in moving "into" their hitbox. but a quick cross-compat test in a wad i made a while ago shows that there's just something wrong with counting their bounding boxes in general and it's direction related (the rooms are facing different directions), not a difference between vanilla and boom.

The kindest gift the BOOM team could have given us would have been if it were impossible to record demos under BOOM with compatibility turned on :P

This decision and the same decision made in other ports since then has lead to a horrific proliferation of incompatible demos with the necessity to not only maintain vanilla behaviors, but the bizarro quirks of every version of the engine between there and now in two different flavors - acting like vanilla and acting like itself.

As for "mastermind/cyber roaring sound once they get a new target", after a quick test on E3M8 I did not spot any difference. Not confirmed.

I didn't mention it in that previous post but all that stuff was with PrBoom+ on complevel 9. I assumed things like sounds would not be different from the original Boom behavior as this is relevant to gameplay.

dew said:
- dead bodies can pass through things, allowing monster wiggle death slides (building up speed by running against a monster, getting slashed to death and sliding the corpse to exit, potentially through the monster and under an otherwise impassible barrier)

Apart from allowing those tricks, this difference also speeds up Boom running in general, as you almost don't get blocked by dying monsters, while in vanilla they stall you for a bit or force you to move around them. Running through a mass of monsters while pushing forth with a plasma gun or BFG (or any weapon, these are just convenient) is faster than in vanilla, for example.